Smooth-body flow separation over a tapered Gaussian bump – flow field topography and topology

A comprehensive set of experiments were performed to document the separated flow over a three-dimensional (3-D) bump with the purpose of generating a benchmark experimental database useful in validating computational fluid dynamics flow simulations and improving model development. The emphasis of this manuscript is on the 3-D topographical and topological features of the separated flow that forms downstream of the bump and its sensitivity to upstream flow conditions. The bump model geometry was designed to provide well-defined and repeatable smooth-body flow separation conditions that were suitable for both experiments and simulations. The bump had a Gaussian streamwise profile with a constant maximum height equal to 8.5 % of its width over the central 60 % of its span. The remaining 40 % were outboard spanwise portions that gradually taper to zero using an error function profile to minimize tunnel sidewall boundary layer interaction effects. The model was immersed in a canonical turbulent boundary layer that was developed on a suspended flat plate in the Notre Dame Mach 0.6 closed-circuit wind tunnel. To document the effect of the incoming boundary layer thickness on the flow separation, the bump model could be located at two streamwise positions. The measurements of the flow separation region included fluorescent surface flow visualization, wall shear stress using oil-film interferometry, mean and dynamic surface pressure, hot-wire anemometry and planar and stereoscopic particle image velocimetry. It is shown that the surface flow separation topology is characterized by the `owl-face pattern of the first kind’. This flow topology consists of four singular points – two saddle points at the bump centrespan and two foci located at a spanwise-symmetric position. It is shown that the spanwise separation of the twin foci increases with Reynolds number indicating a corresponding increase in the spanwise extent of the flow separation. The two surface foci represent the footprint of vortices that lift off the ramp surface and form an arch vortex time-mean off-surface flow topology aft of the bump.